In rodents, sexual stimulation induces a positive affective state that is evaluated by the conditioned place preference (CPP) test. Opioids are released during sexual behavior and modulate the rewarding properties of this behavior. Prairie voles (Microtus ochrogaster) are a socially monogamous species, in which copulation with cohabitation for 6h induces a pair bond. However, the mating-induced reward state that could contribute to the establishment of the long-term pair bond has not been evaluated in this species. The present study aimed to determine whether one ejaculation or cohabitation with mating for 6h is rewarding for voles. We also evaluated whether this state is opioid dependent. Our results demonstrate that mating with one ejaculation and social cohabitation with mating for 6h induce a CPP in males, while exposure to a sexually receptive female without mating did not induce CPP. In the female vole, mating until one ejaculation, social cohabitation with mating, or exposure to a male without physical interaction for 6h did not induce CPP. To evaluate whether the rewarding state in males is opioid dependent, the antagonist naloxone was injected i.p. The administration of naloxone blocked the rewarding state induced by one ejaculation and by social cohabitation with mating. Our results demonstrate that in the prairie vole, on the basis of the CPP in the testing conditions used here, the stimulation received with one ejaculation and the mating conditions that lead to pair bonding formation may be rewarding for males, and this reward state is opioid dependent.

Microtus ochrogaster, the prairie vole, has become a valuable rodent model to study complex social behaviors, including enduring social bonding. Previous studies have related pair bonding in the prairie vole with plastic changes in several brain regions. However, the network interaction between these regions and their relevance to such socio-sexual behaviors has yet to be described. In this study, we used resting state functional magnetic resonance imaging (rsfMRI) to explore the longitudinal changes in the functional interactions of a set of brain regions previously associated with pair bonding in prairie voles. Specifically, 32 prairie voles were scanned at baseline, after 24 hours and two weeks of cohabitation and mating with a sexual partner (16 male-female pairs). Between 48 and 72 hours of cohabitation voles showed significant preference for the partner over a stranger, confirming pair bond formation; however, individual variability was observed. Network based statistics showed no sex differences in functional connectivity, but revealed a common network with significant longitudinal changes between sessions, including the anterior cingulate cortex (ACC), anterior olfactory nucleus (AON), dentate gyrus (DG), dorsal hippocampus (dHIP), lateral septum (LS), medial prefrontal cortex (mPFC), retrosplenial cortex (RSC), ventral hippocampus (vHIP), and ventral tegmental area (VTA). Furthermore, baseline functional connectivity in three sub-networks including the ACC, nucleus accumbens (NAcc), basolateral amygdala, and DG (ACC-NAcc-BLA-DG), the medial amygdala (MeA)-ventral pallidum (VP), and RSC-VTA, predicted the onset of affiliative behavior (huddling latency) in the first hours of cohabitation, even before sexual experience. Finally, a relationship was found between the strength of the pair bond (partner preference index) with long-term changes in the functional connectivity between the ventral pallidum and medial amygdala (MeA-VP). Overall, our findings revealed the association between network-level changes and the process of social bonding, and provide a novel approach to further investigate the neurophysiology of complex social behaviors displayed in the prairie vole.